1. Field of the Invention
The present invention relates to systems used for remote gaging of storage tanks used in storage and processing of petroleum and chemical products, paint, ink, foodstuffs, and the like, and to remote gaging networks using communications protocols for data telemetry.
2. Description of the Related Art
Data telemetry applications for remote sensing of storage tanks in refineries, tank farms, and the like are known in the art. A typical sensing system has a level gauge float or similar sensor connected to a gaugehead. The sensor drives the gaugehead, and the gaugehead provides a local visual indication of the level of liquid in the storage tank. A level encoder/transmitter unit may be mechanically coupled to the gaugehead to translate the mechanical movement of the float sensor/gaugehead assembly to a positionally-encoded electrical signal and to transmit the signal to a remote collection or control system. Other sensors may additionally be provided to measure tank parameters such as temperature, etc.
FIGS. 1A and 1B show a typical float-gaugehead installation in a storage tank as known in the prior art. In this system, float 258 has guide eyes 260 mounted on its edges. Guide wires 262 are fixed to the roof 264 of the tank with roof mounts 266 and to the bottom 268 of the tank with bottom mounts 270 and maintain float 258 on a substantially vertical axis so that it may move up and down in response to changing surface levels of fluid in the tank.
As can be seen from front view 1A and side view 1B, float 258 is generally discus-shaped. Float 258 may be made of any sufficiently buoyant material which is non-reactive with the tank contents. A hollow stainless steel float approximately 14.5 inches in diameter with a 17 inch distance between guide eye centers has been found to be advantageous in this application. For applications where caustic chemicals are handled, the float may be advantageously made of a relatively inert material such as Monel or Carpenter 20.
Eyelet 272 is mounted on the top of float 258 and connected to measuring tape 274 via coupler 276. Measuring tape 274 passes through tank roof 264, sheave elbows 278 and their associated piping, and into gaugehead 10 at primary tape opening 280. Advantageously, measuring tape 274 may be constructed from a stainless steel tape having precision perforations to engage pins on a sprocket in gaugehead 10.
The float need not be freely moveable over the entire height of the tank, but can be disposed in a float well. One type of float well commonly known in the art is shown in FIG. 2. FIG. 2 shows a fixed float well 284. Fixed float well 284 encloses float 258 and stabilizes it within a limited area, thereby obviating the need for guide wires. The fixed float well is mounted to tank wall 286 by float well mount 288 so that the range of travel of float 258 within fixed float well 284 accommodates the tank levels desired to be measured.
Another type of float well used in the art is shown in FIG. 3. FIG. 3 shows a floating float well 290 used in a floating roof tank. In this system, floating roof assembly 292 comprises float well 290, buoyancy chamber 294, and floating roof sealing element 294. Buoyancy chamber 294 causes floating roof assembly 292 to float on the surface of liquid in the tank as it rises and falls, and floating roof sealing element 294 maintains a seal between the tank interior and the tank's external environment to prevent contamination of the liquid. Due to the relatively large mass of floating roof assembly 292, it is somewhat insensitive to changes in the liquid level; however, float 258, which is freely moveable within float well 290, is able to react to small changes in the liquid level, thereby permitting a precise measurement of the level.
Of course, the float system may be used in a wide range of tank shapes. Further, gaugehead 10 may be mounted atop the tank and measuring tape 274 fed into it via auxiliary tape opening 282, thus obviating the need for sheave elbows 278 and their associated piping.
In some applications, it is desirable to maintain float guide eyes 260 above the surface of the tank liquid. In other applications, it is necessary to keep float guide eyes 260 below the liquid surface. Consequently, two versions of float 258 are required. One version, shown in FIGS. 1-3 has guide eyes 260 offset in a direction opposite that of eyelet 272 so guide eyes 260 will be submerged when float 258 is on the liquid surface. The other version has guide eyes 260 offset in the same direction as eyelet 272 so they will always be above the liquid level. The need for two different types of float for this application has resulted in unnecessary complexity of the gaging system.
FIG. 5 shows a Varec Model 2500 Automatic Tank Gauge, which is a typical gaugehead and encoder/transmitter system according to the prior art. Gaugehead 10 includes sprocket wheel 14 which engages holes in measuring tape 274 fed into gaugehead 10 through primary tape opening 280 as shown in FIG. 6, or alternatively, through secondary tape opening 282. Gaugehead 10 has a level meter face 16 fixed on one side. The sprocket/meter face assembly is driven to provide a tank level indication by way of viewing window 20. The calibration of the gaugehead meter may be checked with calibration knob 22.
The gaugehead may be used in a stand-alone configuration to provide level readings visible at the tank site. Level encoder/transmitter 24 may be mounted on gaugehead 10 to provide a data telemetry capability. FIG. 6 shows a typical method of coupling level encoder/transmitter 24 to gaugehead 10. In this arrangement, encoder/transmitter body 26 is mounted on gaugehead body 18 reverse to viewing window 20. Slotted drive coupling 28 links shaft 30 of level encoder/transmitter 24 to a drive pin 32 eccentrically located on sprocket wheel 16 of gaugehead 10. In this manner, when sprocket wheel 16 is driven by level sensor input 12, rotational motion is imparted to encoder/transmitter shaft 30. This rotational movement of shaft 30 is then converted into an information-bearing electrical signal as will be more fully described below, and the resultant signal is presented at transmitter output 34 connected to data cable 36.
The level encoder/transmitter may provide an analog or digital output signal. FIG. 7 is an exploded view of an analog level encoder/transmitter such as the Varec Model 8200 Current Output Transmitter or a similar unit. In this device, encoder/transmitter shaft 30 is coupled to encoder shaft 38 mounted on encoder assembly 40 by a worm gear arrangement. A potentiometer is coupled to encoder shaft 38 so that rotational motion imparted to encoder/transmitter shaft 30 is transmitted to the potentiometer, thus varying the resistance of the potentiometer.
Calibration disk 42 is also coupled to encoder shaft 38. Calibration disk 42 has a scale inscribed around its periphery which is used to place the shaft/potentiometer assembly in arbitrary positions for calibration purposes. Limit switches may be provided on limit switch mounting plate 44 to provide indications when calibration disk 42 (and thus the tank liquid level) are at extreme high and low positions.
The potentiometer output is provided to transmitter board 46 by way of connector cable 48. Transmitter board 46 converts the voltage-varying signal produced by the potentiometer to a current-varying signal using, for example, a linear integrated circuit. The signal is then transmitted to a remote device by way of transmitter output 34 and data cable 36, as shown in FIG. 5. The entire encoder/transmitter is enclosed by cover 50 mounted to encoder/transmitter body 26.
The signal produced by the transmitter may conform to any generally accepted standard. It has been found that output signals directly proportional to liquid level and varying from 4-20 mA or from 10-50 mA provide an acceptable level of compatibility with other devices. Also, the transmitter may be designed to provide an output signal increasing with an increase in tank level; alternatively, it may provide an output signal decreasing with an increase in tank level.
As noted above, the level encoder/transmitter may alternatively provide a digital output signal. FIG. 8 is an exploded view of a digital level encoder/transmitter such as the Varec Model 1900 Digital Transmitter or a similar unit. In this device, encoder/transmitter shaft 30 is coupled to encoder disk assembly 52 mounted on encoder mechanism 54 so that rotational motion imparted to encoder/transmitter shaft 30 is transmitted to encoder disk assembly 52.
Encoder disk assembly 52 may be a brush-type encoder disk. In this arrangement, two non-conductive disks mounted on encoder/transmitter shaft 30 each has one or more metallic traces laminated on it. When the disks are rotated by encoder/transmitter shaft 30, the traces pass under brushes touching the disk surfaces, thus alternately forming and breaking electrical connections. The make-break actions thus provide positional information on the tank level.
Alternately, encoder disk assembly 52 may be an optical-type encoder disk. In this arrangement, two black metal disks each having a code pattern cut therethrough are mounted on encoder/transmitter shaft 30 and are disposed between an array of light emitting diodes (LEDs) on one side of the disks and an array of photodetectors such as phototransistors on the other side of the disks. When the disks are rotated by the encoder/transmitter shaft 30, they alternately form and break an optical path between the LEDs and the photodetectors. This action provides positional information on the tank level. The coding pattern on either type of disk may be designed to provide level information in English units or metric units.
Calibration disk 42 is also coupled to transmitter/encoder shaft 38 through a gear arrangement. Calibration disk 42 has a scale inscribed around its periphery which is used to place the encoder disk in arbitrary positions for calibration purposes. Limit switches may be provided on limit switch mounting plate 56 to provide indications when encoder disk 52 (and thus the tank liquid level) are at extreme high and low positions.
The output of the encoder disk brushes or optical sensors is provided to CPU board 58 by way of CPU connector cable assembly 60 which mates with encoder input connector 62 on CPU board 58. CPU board 58 is powered by power supply board 64 and converts the signal produced by the brushes or sensors to a digital signal which is then presented at CPU output connector assembly 66 and transmitted to a remote device by way of transmitter output 34 and data cable 36. Condulet junction box 68 may be provided on encoder/transmitter housing 26 to provide a convenient means of terminating signal conductors, sensor inputs, and power cables. The entire encoder/transmitter is enclosed by cover 50 mounted to encoder/transmitter body 26.
The signal produced by the transmitter may conform to any generally accepted standard. One format that has been found to work particularly well in environments where tank gaging is commonly used is the Varec digital mark/space pulse code. The mark/space pulse code is implemented on a four-wire field interface. Two wires, B+ and B-, provide power to the encoder/transmitter, while the other two wires, Mark and Space, carry tank level information. Each of these lines is normally held at +48 VDC and dropped to 0 VDC to indicate a mark (on the Mark line) or a space (on the Space line). The interface is idle when both lines are high, and the interface is in a fault state when both lines are low.
Mark/space communications must conform to pulse timing constraints to ensure reliability and accuracy in data communications. FIG. 9 illustrates the timing considerations required in performing mark/space communications at two differing communications speeds as implemented in the Varec Model 1900 Digital Transmitter.
Using the four-wire field interface as described above, a messaging protocol may be implemented between the transmitted and a remote acquisition device. FIG. 10 shows the structure of the messaging protocol implemented in the Varec Model 1900 Digital Transmitter. To initiate a gaging operation, the remote acquisition device sends a poll message on the four-wire interface. The interrogation message is a 16-bit message as depicted in FIG. 11 and consisting of the following bits:
______________________________________ Bit(s) Interpretation ______________________________________ 1 start bit (always Mark) 2 unused 3,4 optional 5-8 most significant device ID bits (BCD ID .times. 100) 9-12 next most significant device ID bits (BCD ID .times. 10) 13-16 least most significant device ID bits (BCD ID .times. 1) ______________________________________
In response to the polling message, the device having an identification number matching the one in the polling message may transmit a 40-bit response message as depicted in FIG. 12 and consisting of the following bits:
______________________________________ Bit(s) Interpretation ______________________________________ 1 start bit (always Mark) 2,3 unused 4-7 most significant device ID bits (BCD ID .times. 100) 8-11 next most significant device ID bits (BCD ID .times. 10) 12-15 least most significant device ID bits (BCD ID .times. 1) 16 reference bit (always Space) 17-37 level data 37,38 optional 40 parity bit ______________________________________
In the response message, the parity is set to odd Mark parity; that is, when the number of Mark pulses in bits 1-39 of the message is even, the parity bit is a Mark. When the number of Mark pulses in bits 1-39 of the message is odd, the parity bit is a Space. Thus, the parity bit ensures that the number of Mark pulses in every message is odd, and if the remote acquisition device receives a response message with an even number of Mark pulses, a fault condition has occurred.
For enhanced reliability, the level data in the response message is transmitted in a reflected Gray code format, in which there is only one bit change for a single digit change of measurement data. FIG. 13 shows a decimal English measurement system code sequence with a range of 00.00 to 79.99 feet in increments of 0.01 feet. Note that in the 0.01 foot increment column, there is only one element change as the level increases from 0.00 to 0.09. Over an 80-foot range, however, this code is required to repeat 8,000 times. Under these conditions, 0.09 and 0.00 become adjacent increments. The code for these increments must still satisfy the rule for only one code element change between adjacent increments. This would not be difficult if only the FTx0.01 column were involved. The FTx0.1, FTx1 and FTx10 columns, however, also come into play.
The above conflict is resolved by using the code itself to determine the code elements. In the FTx0.01 column in FIG. 13, when the level has reached 0.09, the code begins to repeat the preceding in a reflected format. This results in 0.09 being the same as 0.00. The reflected portion of the code is labelled as ODD ad refers to the next significant digit--in this case, the FTx0.1 column.
Zero is considered to be an even number as are 2, 4, 6 and 8. All other numbers are considered as odd. Note that the requirement that only one code element change between adjacent terminals is still satisfied. When the level reaches 0.19, the code is 0001 0000 and has reached a point where 0.09 (odd) is the same code as 0.00 (even). When the level reaches the next increment, only one code element changes (#11) from 0 to 1, and the code reads 011 0000. The 0.1 code is now even, and the 0.01 code now used is also even. The similarity applies to the relationship between FTx0.1 and FTx1.0. The even and odd groups for the FTx0.1 code refer to the odd and even conditions of FTx10 code, and the even and odd groups for the FTx1 code refer to the odd and even conditions of FTx10 code. This coding principle also applies when the level increments are 1 mm in the metric system, as shown in FIG. 14, or in a fractional coding system, as shown in FIG. 15.
The above-described prior art encoders are absolute level encoders; that is, they sense an absolute tank level corresponding to a given float sensor output. Incremental level encoders are also known in the art. These encoders sense a change in the tank level corresponding to a given change in the output of the float sensor. If a starting reference level is known, incremental sensing information may be used to determine the tank level position; however, it has been found that incremental encoders are more sensitive to errors resulting from transient voltages, excessive speed, stray light, power losses and the like.
Another tank gaging parameter often monitored is the temperature of the liquid in the tank. While accurate and precise measurement of the liquid temperature is often necessary for process control procedures, it is sometimes difficult to obtain reliable temperature measurements due to errors induced by internal component and ambient circuit factors.
Some traditional circuits based on resistive temperature devices (RTDs) use potentiometers to compensate for measurement errors present at room temperature. Such techniques involve critical and time-consuming calibration procedures which do not entirely compensate for the measurement errors.
Another compensation technique known in the art involves mathematical modeling with automated equipment to characterize the measurement errors and then nullifying the errors predicted by the model with software parameters. This method again requires expensive equipment and is similarly labor-intensive. Moreover, the temperature measurement device's firmware must be reprogrammed or replaced each time the circuit is repaired or replaced. Thus, the software compensation technique is unsuitable for low volume production applications.
The encoder/transmitter described above may communicate with a remote data acquisition device via any generally recognized standard. One protocol that has been developed for process control applications such as this and which is well-known in the art is the RS-485/MODBUS protocol developed by Modicon. Another appropriate standard is the Varec Mark/Space protocol previously described. Using one of these protocols, data from the transmitter may be sent over long distances to a central acquisition station. With long wiring runs such as these, the gaging system is prone to lightning strikes and other induced transients; therefore, some type of lightning and surge protection is necessary for large distributed gaging systems.